Adds safe_mode integration documentation (#11977)

source/_integrations/safe_mode.markdown

@@ -0,0 +1,25 @@
+---
+title: Safe Mode
+description: Allows Home Assistant to start up in safe mode.
+ha_category: []
+ha_release: 0.105
+logo: home-assistant.png
+ha_quality_scale: internal
+---
+
+The `safe_mode` integration is an internally used integration by the
+Home Assistant Core.
+
+You don't have to configure it in any way since it is automatically always
+available when Home Assistant needs it.
+
+If, during startup, Home Assistant has problems reading your configuration,
+it will still continue to start using bit and pieces from the configuration
+of the last time it did start.
+
+When this happens, Home Assistant will start in "Safe mode" using this
+integration. In this mode, nothing is loaded, but it does give you access to
+the Home Assistant frontend, settings and add-ons.
+
+This gives you the possibility to correct the issue and restart Home Assistant
+to re-try.